It is known that particulate matter (PM), which has carbon as its main component, and ash, which is made of uncombusted components, are generally present in the exhaust gas discharged from an internal combustion engine and are a cause of atmospheric pollution. Due to this, regulations on the amount of particulate matter emissions—along with those on the components present in exhaust gas, e.g., hydrocarbon (HC), carbon monoxide (CO), and nitrogen oxides (NOx)—are becoming more severe year by year. Technology for the collection and removal of this particulate matter from exhaust gas has thus been proposed.
For example, a particulate filter may be disposed in the exhaust passage of an internal combustion engine in order to collect the particulate matter. For example, while being less than that for diesel engines, gasoline engines do discharge a certain amount of particulate matter in their exhaust gas and as a consequence a gasoline particulate filter (GPF) is in some cases installed in their exhaust passage. Particulate filters having what is known as a wall-flow structure are known here; this structure has a substrate constituted of a large number of porous cells wherein the entrances and exits of the large number of cells are plugged in alternation (Patent Literature 1, Patent Literature 2). In a wall-flow particulate filter, the exhaust gas that has flowed in through a cell entrance passes through the interposed porous cell partition and is discharged to and through a cell exit. While the exhaust gas is passing through the porous cell partition, the particulate matter is collected within the pores in the interior of the partition.